Contamination of metal-plating baths by impurity-metal ions is a common problem in the plating industry. One source of the contaminants is the metal parts being plated. Oxidation of the surface layers of these parts during surface cleaning can lead to dissolution of metal ions from the parts and into the plating solution. Contamination also arises from adherence of previous plating solution to the surface of parts that are to be further plated.
A notable example is copper and zinc contamination of nickel-plating baths, in both electrolytic and electroless plating. Concentrations of only about 20 ppm and less of these contaminating metals adversely affect plating quality and so are generally regarded as unacceptable. Iron contamination of nickel-plating baths is also common, although iron concentrations of up to 100 ppm can be reached before there is a serious effect on nickel-plating quality if water soluble ion-chelating compounds are added to the plating solution.
It is exceedingly difficult to remove contaminating metal ions from electroplating solutions without also removing large amounts of the metal being plated. With nickel-plating again as the example, the principal methods of removing copper and zinc contaminants from electrolytic nickel-plating solutions have been variations of a basic method known as "dummying," wherein, for example, a "dummy" cathode with a corrugated surface is placed in the bath and the current density is reduced to very low levels to preferentially plate out the unwanted copper and zinc onto the cathode, which is eventually discarded. Dummying as a decontamination technique has inherent disadvantages, however. It has extremely poor selectivity for copper and zinc over nickel, removing 20 to 500 times as much nickel as copper or zinc, thus requiring replacements of substantial amounts of nickel in the plating bath. Because of the very low current densities required, dummying is an inherently slow process, typically requiring up to sixteen hours of downtime, during which plating of parts cannot be accomplished, and so productivity is lost.
Iron is usually removed by filtration of the solution when it begins to precipitate from the bath as iron hydroxide. However, it would be desirable to remove the iron as an ion before it precipitates, since the presence of iron hydroxide in the plating solution can cause degradation in plating quality.
A possible method for removing trace metal-ion impurities from nickel-plating baths is with the conventional ion-exchange materials. Such a method would have an advantage over dummying in that it could be used simultaneously with the plating of parts, thereby eliminating the loss of productivity associated with dummying. Unfortunately, conventional ion-exchange resins are not sufficiently selective, and a major disadvantage of dummying--loss of nickel from the bath--would still exist.
Another possible method of simultaneously removing trace metal-ion impurities from nickel-plating baths while parts are being plated is with organic liquid ion-exchange agents. These agents can be highly selective, and their use in the removal of metal ions from aqueous solutions is known. In U.S. Pat. No. 3,682,589 to Moore, there is disclosed the selective removal of copper, nickel, iron and cobalt from concentrated zinc sulphate solutions by the use of oxime complexing agents adsorbed onto activated charcoal. Wallace, in U.S. Pat. No. 4,108,640, describes the hydrometallurgical separation of nickel from cobalt by liquid-liquid extraction with organic complexing agents. In Hydrometallurgy 3(1976)65, Kauczor et al. disclose the removal of zinc from cobalt sulphate solutions by the use of a phosphoric acid ester-containing isotropic styrene-divinyl-benzene copolymer resin. In Int. Chem E. Sym., Series No. 42, Kroebal et al. describe recovery of uranium from nitric acid solution with tributylphosphate in Levextrel.RTM. resin. Warshawsky discusses the recovery of zinc, copper, and uranium from hydrometallurgical solutions with similar resins in Trans. Inst. Min. Metall. (Section C: Mineral Process. Extractive Metall.) 83 (1974). However, no suggestion of metal-ion contaminant removal from metal-plating baths with liquid ion-exchange agents has been made in prior work, either by liquid-liquid extraction or with the agent held in microporous media.
There are several possible reasons for this omission. One is that the conventional method of controlling the selectivity of organic liquid ion-exchange agents for one metal ion over another is to adjust the solution variables such as ionic strength, pH, and temperathure. However, in plating solutions these variables must be maintained within a narrow range to permit high-quality plating. There are also potential drawbacks to using the agents in conjunction with plating baths. Organic additives in plating baths which act as plating brighteners can be extracted into the organic agent phase and thus cause degradation in plating quality. Also, problems may arise due to loss of the liquid ion-exchange agent itself. This is particularly true in the case of nickel-plating baths in which organic compounds in the solution (other than brighteners) can cause plating defects such as darkened plate or pitting, and so great care must be taken to avoid such contamination. If, however, these obstacles could be overcome, thereby permitting advantage to be taken of the high selectivity of the organic liquid ion exchange agents, their use would represent a substantial improvement to currently practiced methods of removing metal-ion contaminants from plating baths.